1. Field of the Invention
The invention relates generally to ophthalmological examination instruments and more particularly to fixation targets for attracting and holding the gaze of a patient.
2. Description of Prior Art
In order to perform certain opthalmological procedures such as tonomet, foreign body removal, and fundus photography, it is desirable to fix the eyes of the patient in the approximately straight ahead gaze position. Various ocular fixation devices have been disclosed which are used to attract and hold the visual attention of the patient so as to orient and fix the eyes in the desired gaze portion. Such devices usually include a target such as a light, or animated object upon which the patient is instructed to direct his vision. An example of a typical ocular fixation device is disclosed in U.S. Pat. No. 4,093,359 (Ketcham). The Ketcham invention comprises an animated fixation target such as a toy animal having movable body parts whose movement can be controlled by the examiner, so as to hold the visual attention of the patient.
The most common type of prior art ocular fixation devices centers around the use of a single movable light target, such as the one disclosed in U.S. Pat. No. 3,897,141 (Schocket). The Schocket device comprises a small tight adapted to be moved in a circular path around and above the patient's face. The Schocket device is typical of fixation devices which are currently in wide use among ophthalmologists. A further example of a related fixation device is found in U.S. Pat. No. 3,871,753 (Papritz), which also employs a single light target which is movable.
U.S. Pat. No. 3,484,155 (Praeger) discloses the use of a movable primary illuminated target and an ancillary stationary illuminated target, both of which are mounted on the head of the examiner. One disadvantage common to all of the prior art devices is that their operation requires repositioning of an illuminated target. The transmission of electrical energy to the movable target requires the use of wipers, brushes, slip rings, and the like. It is well known that these movable electrical contacts significantly reduce the reliability of the devices and greatly increase their cost. Even the Praeger device which discloses the use of two illuminated targets employs a wiper assembly for transmitting electrical power to the movable primary target.
All of the prior art devices require a repositioning of a fixation target during the normal course of their operation. Another major disadvantage in the aforementioned devices is that the supporting structure for the movable fixation target can interfere with or even collide with the patient or the examination equipment. The cords, wiper arm and slip ring assemblies, and other complex structural components of the prior art devices detract from their reliability and usefulness.